Improved radiant burner

ABSTRACT

A radiant burner comprises a body defining a premixing chamber and a combustion chamber. The premixing chamber is separated from the combustion chamber by at least one radiant burner plate ( 2 ) which has multiple levels of burner surface. The combustion chamber is further limited by a first radiant screen ( 4 ). The radiant burner further comprises a second radiant screen ( 3 ) in the combustion chamber. The second radiant screen is spaced from, but near and parallel to the radiant burner plate(s), such that this second radiant screen acts as an extended burner surface and also heats up said at least one radiant burner plate when in use.

TECHNICAL FIELD

The present invention relates to radiant burners comprising a radiantburner plate and a screen.

BACKGROUND ART

Radiant burners comprising a radiant burner plate and a screen are knowne.g. from U.S. Pat. No. 4,799,879 or EP0539279. The screen together withthe radiant burner plate provides the radiative output of the burner,which averages at levels around 50% efficiency. In the past theradiative output of the burners has been increased by modification ofthe radiant burner plate from a radiant burner plate with rows ofthrough holes or perforations serving to channel the mixture of air andcombustion agent from the rear of the plate to the radiating face, to aradiant burner plate wherein the through holes or perforations arearranged in what is nowadays called honeycomb pattern as described ine.g. U.S. Pat. No. 4,569,657 or U.S. Pat. No. 4,799,879. This or similarmodifications of the radiant burner plate increased the temperaturelevel and consequently also the radiative output of the burner. On theother hand, these honeycomb-like patterns are creating local overheatingof the burner plate on the places where the flames are, and also causepoor temperature uniformity and relative low average burner surfacetemperature and thus lower energy efficiency. These local hightemperatures define therefore also the limitation of the use of suchthrough hole or perforation patterns, and also define the limitation onthe amount of radiation energy which can be obtained with such systems.

Another way of achieving higher radiative output was proposed in e.g.U.S. Pat. No. 3,847,536 which uses two radiative screens above theradiant burner plate. Also this modification of the radiant burnercaused local overheating of the radiant burner plates in the middle ofthe radiant burner, which urged the skilled person to lower inputs whichresulted in lower (local) temperatures of the radiant burner plate forprolonging the life time of the radiant burner.

However, still further enhanced efficiency of the radiant burners isdesired.

DISCLOSURE OF INVENTION

An aspect of the claimed invention provides a radiant burner whichcomprises a body defining a premixing chamber and a combustion chamber.The premixing chamber is separated from the combustion chamber by atleast one radiant burner plate which has multiple levels of burnersurface. The combustion chamber is further limited by a first radiantscreen. The radiant burner further comprises a second radiant screen inthe combustion chamber. The second radiant screen is spaced from, butnear and parallel to the radiant burner plate(s), such that this secondradiant screen acts as an extended burner surface and also heats up saidat least one radiant burner plates by back radiation when in use. In apreferred embodiment, the second radiant screen is an arrangement ofparallel spaced round rods or square bars. In a preferred embodiment,first and second radiant screens are produced from highly heat resistantmaterials such as ceramics, especially aluminium or zirconium oxide,aluminium titanate, silicon oxide, corundum or mullite, silicon carbide,silicon nitride or metal infiltrated ceramics, such assilicon-infiltrated silicon carbide. Alternatively, the radiant screenscan also be fabricated from heat-resistant materials of other naturesuch as e.g. materials which contain more than 50% by weight of a metalsilicide, such as molybdenum disilicide (MoSi₂) or tungsten disilicide(WSi₂). In another preferred embodiment, the radiant screens arefabricated from highly heat resistant steel grades, such as high levelstainless steel grades like Kanthal APM or APMT, different grades ofFeCrAl alloy designed for high temperature corrosion, Chrome/Nickelsteel grades like Avesta 253 MA, 153 MA, Inconel 601, Incoloy 800HT,Incoloy MA956.

The radiant burner plate is preferably made of a ceramic material withhigh temperature resistance, and excellent mechanical and thermodynamicproperties such as e.g. cordierite or zirconia; partially stabilisedzirconia (PSZ), alumina, silicon carbides or other high level technicalceramics. Height difference in between two levels of burner surface ofthe radiant burner plate is preferably from 1 to 20 mm. More preferably,from 1 to 10 mm. Even more preferably, from 2 to 7 mm. Most preferably 5mm.

The radiant burner plate has multiple levels of burner surfaces. In apreferred embodiment, these multiple levels are arranged in rows and arealternating per one row of through holes/perforations on the radiantburner plate. An example of such burner plate can be found in FIG. 1, oralternatives in FIGS. 2 and 3. These types of burner plates, as such,provide less emissivity compared to ceramic tiles with honeycomb orsimilar perforation patterns. This is due to the multiple level burnersurface, wherein the lower levels of the burner surface of the radiantburner plates provide a higher radiative output because the sides of therows also heat up and provide an additional radiative output, but thehighest level of burner surface does not have such additional radiativeoutput. So the overall radiative output, and therefore also the energyefficiency, of such multilevel radiant burner plate as such, is lowerthan honeycomb-like perforations in the radiant burner plate.

However, although radiant burner plates are used which as such have alower radiative output, it was surprisingly observed that by the use ofsuch a second radiant screen near the radiant burner plates, theradiative output of the radiant burner plates can be increased withoutleading to local overheating of the burner plates, as this would resultin early failure of the radiant burner plates. This might be explained,without pretending to be scientifically correct, by the fact that theback radiation of the second radiant screen on the radiant multilevelburner plates is the highest on the highest level of the burner surfaceas this is closest to the second radiant screen. This highest levelthereby also heats up more than the lower levels of the burner surface,which are at a bigger distance from this second radiant screen. As theselower levels in the burner surface of the radiant burner plates werealready at higher temperatures by the effect of the flames heating upthe surface surrounding the cavity wherein the perforations open, theoverall effect of the present invention is that the different levels inthe burner surface of the radiant burner plates are at the sametemperature when in use. Stated otherwise, a greater temperatureuniformity of the burner surface of the radiant burner plate isattained. The person skilled in the art will understand that thisgreater temperature uniformity combined with the plurality of radiantscreens results in a significant higher energy efficiency of thecomplete radiant burner. In a preferred embodiment, the distance betweenthe second radiant screen and the highest level of burner surface of theat least one radiant burner plates is between 3 and 50 mm. Morepreferably, the distance between the second radiant screen and highestlevel of the radiant burner plate is between 5 and 30 mm, even morepreferably between 10 and 25 mm, most preferably between 15 and 20 mm.In a preferred embodiment, the second radiant screen is positioned suchthat the second radiant screen follows the direction of the rows of thehighest level of burner surface of the radiant burner plate.

The first radiant screen is preferably a metal grid. In anotherpreferred embodiment, the first radiant screen is an arrangement ofparallel spaced round rods or square bars. More preferably, the firstand second radiant screens are made of an arrangement of parallel spacedround rods or square bars. In a further preferred embodiment, the firstand second radiant screens are arranged in the same direction. In analternative preferred embodiment, the first and second radiant screensare arranged in shifted angles with respect to one another. Morepreferably, the first and second radiant screens are at a 90° angle.

A further observed advantage of the present invention is a lower levelof emissions of byproducts of combustion, such as Nitrogen Oxides orCarbon Monoxide, which is probably due to the second radiant screenwhich acts as an extended burner surface and provides a more completecombustion of the gas-air mixture.

Another aspect of the claimed invention provides a radiant burner withat least one further radiant screen in the combustion chamber.

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

Example embodiments of the invention are described hereinafter withreference to the accompanying drawings in which

FIGS. 1 to 3 show a cross section of example embodiments of radiantburner plates used in the present invention.

FIG. 4 shows an example embodiment of the present invention, with cutout for better view of the build up of the radiant burner.

FIG. 5 shows a side view of the example radiant burner of FIG. 4, alsowith cut out for better view of the build up of the radiant burner.

FIG. 6 shows an alternative example embodiment of the present invention.

FIG. 7 shows a side view of the example radiant burner of FIG. 6.

MODE(S) FOR CARRYING OUT THE INVENTION

Example embodiments of the present invention will now be described withreference to FIGS. 1 to 7.

FIGS. 1 to 3 show cross sections of example embodiments of radiantburner plates which might be used in the present invention. FIG. 1 showstwo levels of burner surface of the radiant burner plate 2, FIGS. 2 and3 show three levels of burner surface, in two alternative forms.

FIGS. 4 and 5 show an example embodiment of the present invention. Thefirst radiant screen 4 is a highly heat resisting metal grid fabricatedfrom highly heat resistant steel grades, such as high level stainlesssteel grades like Kanthal APM or APMT, different grades of FeCrAl alloydesigned for high temperature corrosion, Chrome/Nickel steel grades likeAvesta 253 MA, 153 MA, Inconel 601, Incoloy 800HT, Incoloy MA956. Thesecond radiant screen 3 is made of a highly heat resisting ceramicmaterial, in this example aluminium or zirconium oxide, aluminiumtitanate, silicon oxide, corundum or mullite, silicon carbide, siliconnitride or metal infiltrated ceramics, such as silicon-infiltratedsilicon carbide with a silicon infiltration grade of 5 to 50% or evenmore. Alternatively, the radiant screens can also be fabricated fromheat-resistant materials of other nature such as e.g. materials whichcontain more than 50% by weight of a metal silicide, such as molybdenumdisilicide (MoSi₂) or tungsten disilicide (WSi₂). The radiant burnerplate 2 is made of a two level burner surface, ceramic tile made ofcordorite or alternate thermodynamically suited ceramics as mentionedabove.

FIGS. 6 and 7 show an alternative example embodiment of the presentinvention. The first and second radiant screens are made of highly heatresisting material, in this example a ceramic like aluminium orzirconium oxide, aluminium titanate, silicon oxide, corundum or mullite,silicon carbide, silicon nitride or metal infiltrated ceramics, such assilicon-infiltrated silicon carbide with a silicon infiltration grade of5 to 50% or even more. Alternatively, the radiant screens can also befabricated from heat-resistant materials of other nature such as e.g.materials which contain more than 50% by weight of a metal silicide,such as molybdenum disilicide (MoSi₂) or tungsten disilicide (WSi₂). Inthis example this first and second radiant screens are arranged indirections which are 90° with respect to one another. The radiant burnerplate 2 is made of a two level burner surface, ceramic tile made ofcordierite.

Thus there has been described a new radiant burner 1 possessing greatflexibility of use and which is capable of reaching temperatures ofabout 1300° C. with a considerable radiation factor increase of about10% compared to existing technology.

Because of their possible use at very high temperatures e.g. 1300° C.and higher, their high energy efficiency and their long service life,the radiant burner of the present invention are particularly suitablefor drying web materials at high web speeds. One preferred area ofapplication is the drying of moving paper webs.

The new improved radiant burner comprises a body defining a premixingchamber and a combustion chamber. The premixing chamber is separatedfrom the combustion chamber by at least one radiant burner plate whichhas multiple levels of burner surface. The combustion chamber is furtherlimited by a first radiant screen. The radiant burner further comprisesa second radiant screen in the combustion chamber.

The second radiant screen is spaced from, but near the radiant burnerplate(s), such that this second radiant screen acts as an extendedburner surface and also heats up said at least one radiant burner platewhen in use.

1. A radiant burner comprising a body defining a premixing chamber and acombustion chamber, said premixing chamber being separated from thecombustion chamber by at least one radiant burner plate which hasmultiple levels of burner surface, said combustion chamber being furtherlimited by a first radiant screen characterised in that said radiantburner further comprises a second radiant screen in said combustionchamber, said second radiant screen being spaced from, but near andparallel to said at least one radiant burner plate, such that saidsecond radiant screen acts as an extended burner surface and also heatsup said at least one radiant burner plate when in use.
 2. A radiantburner according to claim 1, wherein said second radiant screen is anarrangement of parallel spaced round rods or square bars.
 3. A radiantburner according to claim 1, wherein said first radiant screen is ametal grid or an arrangement of parallel spaced round rods or squarebars.
 4. A radiant burner according to claim 1, wherein said at leastone radiant burner plate is a ceramic burner plate.
 5. A radiant burneraccording to claim 1, wherein said at least one radiant burner plate hasa height difference in between 2 levels of burner surface of 1 to 20 mm.6. A radiant burner according to claim 1, wherein said combustionchamber further comprises at least one further radiant screen.